Until quite recently, the resolution of optical microscopes was limited by the wavelength of the light used. Particles smaller than one-half the wavelength could not be resolved. Transmission and scanning electron microscopy (TEM and SEM) were developed to resolve structures smaller than the wavelength of the visible light, but they were limited by the prerequisite of an electrically conducting sample.
The development of scanning tunneling microscopy allowed the resolution of structures as small as individual atoms. The original scanning tunneling microscope (STM) used tunneling electrons as the signal source within the microscope. Like TEM and SEM, this required an electrically conducting sample to supply the electrons.
There have been many attempts to exploit the near-field radiation of visible light photons. These near-field scanning microscopes (NFSM) directed light onto an aperture of subwavelength size connected to a detector. A near-field was generated at the aperture which interacted with a sample to produce a modulated near-field as the aperture was advanced proximate to the sample. Photons from the modulated near-field would then be detected. Thus, as the aperture moved across the sample, it would sense the modulations of the near-field. A raster scan of the sample by the aperture produced an image of the sample in the scanned area. Variations of this type of microscope have been developed, but in each case, the near-field phenomenon is created adjacent to a subwavelength size aperture and the useful size of the near-field is typically of the order of a wavelength in all dimensions. Because of its small size, this near-field is typically scanned across the sample.
Three methods have been used to maintain the aperture proximate to the sample during scanning. Electron tunneling feedback has been used to maintain aperture to sample separations of less than 1 nm. The method required coating both the sample (if the sample was non-conducting) and the aperture with a thin (approximately 20 nm) layer of a conducting material (e.q. gold). A second method used a constant height mode. The aperture was brought closer and closer to the sample until the desired resolution was obtained. In operation, the method was useful only for flat samples or flat areas of rough samples. The third method used a contact mode. The aperture was advanced until it contacted the surface, a tunneling current of photons was measured, and the aperture was retracted. This procedure was repeated as the aperture was scanned across the sample without any source of feedback. Again, the sample must be fairly flat and capable of withstanding contact with the aperture.
There are many types of samples, for example, biological samples, which may not survive the mechanical pressure of a contact in near-field scanning microscopes or a coating of the sample by a conducting surface. In addition, the techniques that use electron tunneling feedback to maintain proximity to the sample may also require vacuum conditions to best use electron tunneling.
Therefore, it is an object of the present invention to provide a means for high resolution optical microscopy applicable to a variety of specimen types including those that are nonconducting, non-flat, or of delicate structural composition. It is an additional object of the present invention to provide this microscopy under a variety of environmental conditions.